The reason for connecting the 2 analog switches to Vmcu is, I think, that one can power the board, externally, with Vmcu only. In that case the switches wouldn’t be powered otherwise. And since they don’t allow voltage on their inputs above supply voltage, this would be bad.

I did a project recently with the EFM32 (participated in a design contest with it). It was rather simply to get the EFM32 to a low power consumption – but it was rather difficult to do this with the rest of the periphery. The circuit is powered by a LiIon battery – and it’s not that easy to find a LDO with 1 µA quiescent current… (but I still managed to get standby current of the whole circuit down to about 5 µA, and it involves 12 chips)